Relatively small (e.g., 3 to 5 IRE) signals encoding digital information can be admixed together with composite video signals without being readily evident in television pictures generated from those composite video signals if suitable restrictions on the digital signal format are observed. This is pointed out by A. L. R. Limberg, C. B. Patel and T. Liu in their U.S. patent application Ser. No. 08/108,311 filed Aug. 20, 1993, entitled APPARATUS FOR PROCESSING MODIFIED NTSC TELEVISION SIGNALS, WITH DIGITAL SIGNALS BURIED THEREWITHIN, and incorporated by reference herein. The inventions described in U.S. patent application Ser. No. 08/108,311 like the inventions described herein are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreements so to assign inventions made within the scope of employment. U.S. patent application Ser. No. 08/108,311 describes phase-shift-keyed (PSK) modulation of a subcarrier at an odd multiple of one-half scan line frequency with serial-bit digital data supplied at a symbol rate that is a multiple of one scan line frequency. U.S. patent application Ser. No. 08/108,311 indicates a preference for repeating frames of the modulated subcarrier in antiphase in successive pairs of consecutive frames of the NTSC television signal. Such repetition of data in pairs of frames makes PSK subcarrier accompanying the composite video signal detected from the NTSC television signal less apparent in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in a digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal that describes static portions of successive television images. U.S. patent application Ser. No. 08/108,311 also indicates a preference for repeating the modulation of the digital data in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, this providing a basis for using line-comb filtering in the digital signal receiver to separate PSK subcarrier from the chrominance portion of the composite video signal.
Such procedures generate a broadband frequency spectrum overlapping frequency spectrum of the NTSC television signal, but with most of the energy in the former frequency spectrum falling into the so-called Fukinuki "windows" or "holes" in the latter frequency spectrum. To gain an understanding of what these "windows" or "holes" are, the reader is referred to T. Fukinuki et al., "Extended Definition TV Fully Compatible with Existing Standards", IEEE Transactions on Communications, Vol. COM-32, No. 8, August 1984, pages 948-953; and T. Fukinuki et al., "NTSC FULL COMPATIBLE EXTENDED DEFINITION TV PROTO MODEL AND MOTION ADAPTIVE PROCESSING", reprinted from IEEE Communications Society IEEE Global Telecommunications Conference, No. 4.6, Dec. 2-5, 1985, pages 113-117; the disclosures of which are incorporated hereinto by reference. U.S. Pat. No. 4,660,072 issued Apr. 21, 1987 to T. Fukinuki and entitled TELEVISION SIGNAL TRANSMISSION SYSTEM also describes Fukinuki "windows" or "holes" and is incorporated hereinto by reference.
When the NTSC television signal with digital signals buried therewithin is reproduced on the viewing screen of a conventional television set, spectral energy falling into the Fukinuki windows of luminance signal tends not to be visible to a viewer who is at a normal viewing distance away from the screen or is further away. To some extent this is because of adjacent-line averaging effects owing to limits on spatial resolution by the human visual system when viewing the screen from a distance. To a greater extent this because of frame averaging effects owing to limits on temporal resolution by the human visual system and to persistence of the phosphors in the viewing screen. TV sets that use line-comb filtering to separate luminance and chrominance components of composite video signal supplied from the video detector cancel spectral energy falling into the Fukinuki windows of luminance signal, so frame averaging effects need not be relied on as the only mechanism for making the digital video inapparent in the television images as viewed on the screen of a TV set. Top-of-the-line TV sets that use frame-comb filtering can cancel spectral energy falling into the Fukinuki windows of chrominance signal as well as into the Fukinuki windows of luminance signal. However, this spectral energy will appear as color noise in color TV sets that do not employ frame-comb filtering. The desire to keep this color noise reasonably low has been a principal limitation on the permissible amplitude of the signals encoding digital information.
In practice, when using the Fukinuki "windows" or "holes" for the transmission of analog video information, the spatial and temporal correlation/anti-correlation patterns of the additional video information prevent the degree of randomness of signal that is necessary for its being completely hidden in a normal television picture received by TV receivers already in the field, giving rise to so-called "Fukinuki phantoms" in the horizontal spatial frequencies below about 1 MHz in the video detector response. Fukinuki phantoms can exist for PSK subcarriers that amplitude-modulate the VSB AM picture carrier in-phase. However, the likelihood of noticeable Fukinuki phantoms is very low, owing to the low power of these PSK subcarriers and the low probability of repeating digital words at frame intervals, except during forced repetition of data in consecutive frames.
U.S. patent application Ser. No. 08/108,311 describes a suppressed, vestigial-sideband, amplitude-modulated (VSB AM) carrier of the same frequency as the VSB AM picture carrier, but in quadrature phasing therewith, being used to transmit the subcarrier modulated with digital data. This procedure suppresses Fukinuki phantoms to the extent they occupy the band where the VSB AM carriers are DSB AM in character. Transmitting the digital information in VSB AM sidebands of a suppressed carrier that is in quadrature with the VSB AM video carrier permits the transmission of the digital information at relatively low power with less increase in the E.sup.2 /R radiated power from the antenna than would be the case with transmitting the digital information in the VSB AM sidebands of the video carrier.
In each of the digital signal receivers described in U.S. patent application Ser. No. 08/108,311 synchronous detection of the quadrature-phase VSB AM carrier recovers the digital subcarrier, without substantial accompanying composite video signal energy in the baseband extending up to 0.75 MHz frequency. Above 0.75 MHz the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier. The composite video signal is detected with gradually increasing efficiency up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete. Over the same 0.75 to 1.25 MHz frequency range the efficiency with which the digital subcarrier is detected gradually decreases to half its value below 0.75 MHz. A synchronous video detector detecting the quadrature-phase VSB AM video carrier will, providing the intermediate-frequency (IF) amplifier passes the vestigial sideband, generate a response to the PSK subcarrier and remnants of NTSC composite video signal that does not include direct components or synchronizing pulses. This reduces the dynamic range of the synchronous video detector response to the quadrature-phase VSB AM video carrier, easing the problem of digitizing the response without losing low-level PSK subcarriers because of quantization effects.
U.S. patent application Ser. No. 08/108,311 describes cascading of a lowpass line-comb filter and a highpass frame-comb filter following the synchronous video detector for quadrature-phase VSB AM video carrier. The lowpass line-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from chrominance signal portions of the frequency spectrum of an NTSC signal, particularly of an NTSC signal that has been appropriately pre-filtered. The highpass frame-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from motion-free luminance signal portions of the frequency spectrum of an NTSC signal. U.S. patent application Ser. No. 08/108,311 teaches that the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be viewed as the frequency spectrum of a jamming signal accompanying the PSK signal. Accordingly, the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be discriminated against by synchronous symbol detection.
U.S. patent application Ser. No. 08/108,311 advocates the use of binary phase-shift-keying of a PSK carrier, a single-sideband (SSB) of which is selected for translation in frequency to form an upper sideband of a suppressed subcarrier that is at a frequency which is a small odd multiple of one-half scan line frequency. The generation of this SSB BPSK subcarrier introduces complications in the construction of the transmitter, and the detection of this SSB BPSK subcarrier introduces complications in the construction of the digital signal receiver. In the digital signal receiver, there is a need for one or more synchronous detectors for demodulating a PSK subcarrier and one or more oscillators with automatic phase and frequency control (AFPC) for regenerating the unmodulated subcarrier(s) used in synchronously detecting a PSK subcarrier. The phase lock loop used in the AFPC to lock each local oscillator to horizontal sync, color burst, PSK suppressed subcarrier or symbol transitions is prone to problems in regard to maintaining stability of the frequency of oscillations. When upper sideband SSB BPSK is received on a subcarrier, the frequency of which is only a 100 kHz or so, the digital signal receiver uses an upconverter, single-sideband filtering, and a downconverter after the quadrature-phase video detector in order to synchronously detect the BPSK modulation.
These complications and problems are avoided in the inventions described herein by binary phase-shift-keying the quadrature-phase VSB main carrier itself, rather than a subcarrier thereof. In the digital signal receiver the synchronous video detector for quadrature-phase VSB video carrier detects the BPSK modulation directly. The efficiency of this detection is reduced above 0.75 MHz as the BPSK carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier. Over the 0.75 to 1.25 MHz frequency range the efficiency with which the digital subcarrier is detected gradually decreases to half its value below 0.75 MHz, which value of detection efficiency is maintained for frequencies above 1.25 MHz, but below the roll-off of the lowpass filtering establishing detector bandwidth. At the transmitter, the high frequencies of the pulse train used for phase shift keying can be pre-emphasized to compensate for the loss in detection frequency at the digital signal receiver when the BPSK modulation becomes single-sideband in nature.
U.S. patent application Ser. No. 08/108,311 indicates a preference for repeating the BPSK modulation in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, to provide a better basis for separating digital data from interfering chrominance sidebands of the suppressed color subcarrier of the NTSC composite video signal. This retransmission of the BPSK modulation halves the digital transmission rate in the long term; and the attempt to utilize, for digital data transmission, the band of baseband frequencies of the NTSC composite video signal already occupied by chrominance sidebands for digital data transmission generates color noise in most of the color TV sets already in existence. A better system compromise is to narrow the band of baseband frequencies into which digital modulation is introduced for transmission, so it is not co-extensive with the band occupied by the chrominance sidebands, and not to repeat the digital modulation in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal. This sacrifice of bandwidth for transmitting digital information avoids the digital modulation causing color noise in existing color TV sets, and not repeating the digital modulation in antiphase in adjoining scan lines increases the digital transmission rate to make up for the sacrificed bandwidth. In U.S. patent application Ser. No. 08/108,311 the digital modulation repeated in antiphase in adjoining scan lines is lowpass line-comb filtered to double the power of the PSK and improve its capability of withstanding interference from the NTSC composite signal remnants as a jamming signal. To the extent that this loss is significant, it can be made up for by increasing the power of the transmitted PSK (e.g., increasing it from 3 IRE to 4.5 IRE), since the appearance of color noise (which establishes the practical limitation on the power of the transmitted PSK when its frequencies overlap those of chroma) is not a problem with the narrower bandwidth of modulating signal. While there is some increase in the likelihood of impulse noise completely obliterating the reception of the PSK when the digital modulation is not repeated in antiphase in adjoining scan lines, the suppression of impulse noise through error-correcting coding is achieved at a lower overhead cost.
Not having to repeat digital modulation in antiphase in adjoining scan lines, so as to be able to separate digital modulation from interfering chrominance sidebands by line-comb filtering, provides the freedom to design the signal for data transmission such that line-comb filtering can be used to separate digital modulation from interfering luminance signal. Dynamic portions of the television images will not repeat from one frame to the next, so frame-comb filtering will not separate luminance signal descriptive of them from digital modulation that is repeated in antiphase in respective frames of successive pairs of frames, which pairs do not overlap in time. Luminance signal descriptive of these dynamic portions of the television images have a pronounced tendency to repeat at corresponding horizontal positions in successive horizontal scan lines, and so can be discriminated against by highpass line-comb filtering. The modulation of the digital data in contiguous pairs of adjoining scan lines can be repeated, to afford a basis for passing the data through the highpass line-comb filtering without change in the data, but this halves the long-term data rate through the system, without gain of sufficient compensating advantage in return.
Better practice, it is pointed out herein, is to use partial-response filtering in the transmitter of a type in which the digital partial-response filter response recovered in the digital signal receiver as binary digital data will, when supplied as input signal to a highpass line-comb filter, generate ternary or other multiple-level digital data. This procedure does not reduce the long-term data rate through the system.